1. Field of the Invention
The present invention relates to a compressor, and, more particularly, to system and method for controlling a linear compressor, which are capable of controlling a triac to vary a stroke of a piston of the linear compressor.
2. Description of the Related Art
Generally, a linear compressor, whose piston is directly connected to a mover of a linear motor, reciprocates the piston as the motor performs linear motion.
Since a linear compressor does not have a crankshaft, which transforms a rotational motion to a linear motion, frictional loss is relatively small. Therefore, compression efficiency of the linear compressor is greater than that of a general compressor.
Such a linear compressor is operated such that its piston reciprocates based on a voltage, which is applied thereto based on a stroke command set by a user, and thus the stroke is varied to control freezing force. Namely, the stroke is calculated on the basis of the voltage and current applied to a motor of the linear compressor. Afterwards, if the calculated stroke is smaller than the stroke command, a triac, which applies alternating current (AC) to the linear compressor, is operated such that its ON duration is elongated, thereby increasing the voltage applied to the compressor. On the other hand, if the calculated stroke is greater than the stroke command, the triac is operated such that its ON duration is reduced, thereby decreasing the voltage applied to the compressor. Therefore, the stroke cycling distances of the linear compressor are varied by the above-mentioned processes, and thus a freezing force is controlled.
However, the prior art technology has disadvantages in that, although the stroke cycling distance is reduced by a relatively small amount, since the freezing force is rapidly decreased, the frictional loss rate is enlarged when the freezing force is varied, and clearance volume is also increased, thereby decreasing its efficiency. In order to resolve the problems, a prior art technology has been proposed and is described in detail below.
In Korean Patent Application No. 10-2004-0026918 (which was published on Apr. 1, 2004), as shown in FIG. 1, the prior art embodiment includes two windings of a motor, which input direct current (DC) and alternating current (AC), respectively. Also, in Korean Patent Application No. 10-2004-0101764 (which was published on Dec. 3, 2004), a method is adopted which selectively inputs DC and AC on the basis of load. Namely, as shown in (a) of FIG. 2, when AC is applied to the linear compressor under normal conditions (in which load is preset when the linear compressor is designed), since a large force is applied to a piston, the maximum pressing volume is secured. Also, as shown in (b) of FIG. 2, only AC is applied to the linear compressor under a specific load condition (the maximum load), since the shoved amount of the piston is relatively small, the maximum pressing volume cannot be secured. As such, when the maximum pressing volume is not secured, AC and DC are applied to the linear compressor such that the center of the piston cycling is moved to an opposite side of a delivery valve. Therefore, the shoved amount of the piston is increased and the maximum pressing volume can be secured, as show in (c) of FIG. 2.
In the method wherein AC and DC are applied to a linear compressor, pressing efficiency can be designed as the center of the piston cycling is moved, considering load. However, the prior art technology still has disadvantages in that, the efficiency of the motor windings is low and, since motor windings should be additionally installed, the number of assembly processes is increased.
On the other hand, in Korean Patent Application No. 10-2004-0101768 (which was published on Dec. 3, 2004), a method for varying driving frequency of a linear compressor by using inverters to control a stroke is disclosed. As shown in FIG. 3, in order to make the driving frequency coincide with a mechanical resonant frequency as each time load is varied, the driving frequency variable constant is detected as an average value which is obtained by multiplying a stroke by current for a period. Then a driving frequency in a state in which the average value is approximated to ‘0’ is detected, as a driving frequency command value, thereby improving the driving efficiency of the compressor. As such, in order to vary the driving frequency, inverters are used. However, since the controller which controls stroke using the inverters, is expensive, the prior art technology is not cost-effective.
On the other hand, a method which varies stroke based on stroke voltage to control freezing force, has drawbacks in that the center of piston reciprocation is moved according to load conditions when the piston reciprocates. Since the movement distance is increased under overload, the piston and the delivery value collide with each other. In order to resolve this problem, there is a method disclosed in Korean Patent Application No. 10-2002-0041984 (which was published on Jun. 5, 2002). Namely, as shown in FIG. 4, the method involves detecting loads, and controlling phases of triacs according to the loads, changing pressing cycles and expansion cycles. If overload is detected, as shown in FIG. 5, ON durations of the triacs are increased more at the pressing cycle than at the absorption cycle, such that the piston cannot be excessively shoved back. Therefore, a collision between the piston and the delivery valve can be prevented. These technologies should be operated such that an operation unit 300 inputs current of a current detection unit 200, integrates the current for a period, and outputs a work arithmetic signal Wi based on the integration, and a microcomputer 400 reads a pressing difference between absorption and delivery from an absorption/delivery pressing difference storage unit 500 and detects the difference as a load. In order to detect such loads, since current is integrated to operate the work arithmetic signal, the operation processing time is increased, and the pressing difference between absorption and delivery for each work arithmetic signal according to a load condition is experimentally obtained. Also, the obtained difference must be stored, and it is difficult to reflect all of the calculated work arithmetic signals thereto.